An excellent lecture on Tuesday Dave. I'm sure some members, like me, will be giving more thought to a dedicated DSLR. Here's the response to a couple of questions I asked Andy Ellis of Astronomiser. Perhaps other members have a different opinion.

Hi BillThe 40D is getting on a bit now and doesn't include some of the more important recent advances in sensor technology like improved pixel fill and microlensing which a camera like the 1100D does have. Consequently, in terms of the quality of the out-putted picture for astrophotography, the 1100D is a better camera than the 40D.

Personally, I'd recommend (used) 1100D, 600D or 50D and new, either the 1200D or 700D - all are markedly more sensitive than the 40D.

In terms of the mods available, I'd recommend using the camera to astrophotography only and therefore getting a rear filter removal - replacing the filter allows the continued use of the auto focus function, but if you are using the camera for astronomy only, you won't use this.

There are a number of these types of retailers. I got my 450D already astro modified with the box, 2 batteries, charger etc for £230 (IIRC) so you can get them for that sort of money. I took it to Norway & it wrapped on me at -25 degrees C, then i returned it for a replacement absolutely free. Personally, i feel its pointless buying a new camera then shelling out again to get it astro modified - it just invalidates the warranty.

Bill,The more modern the camera the smaller the pixels. Despite Canon always claiming with each new model better mirco-lensing, reduced pixel gaps and increased sensitivity the reality is reduced signal per pixel. So saying the current models are more sensitive than the 40D doesn't tell the full story. The dynamic range of the 40D is slightly higher than the 1100D.

The 1100D was the last with the old sensor mounting technology and is easier/cheaper to convert (see later). With not too small pixels it does represent probably the best value. I am surprised he recommends the 50D as it has a bad reputation for (banding) noise - the 40D was better but not perfect. The 60D and 600D seemed to cure the banding noise issue but are 18 mega-pixels. I currently use a modded 60D - the small pixels are not too much of a problem with short focal lengths.

As regards the recommendation of the 700D - this uses a "hybrid" split-pixel sensor that seems to be optimised for video so I would avoid it until evidence emerges as to whether this compromises use for astronomy.

Lastly, regarding filter removal or replacement. The reference to losing auto-focus is irrelevant and misses the point. If the focus position is maintained either by a replacement filter or moving the chip forward then all lenses will focus at infinity. If this is not done then many/most camera lenses will NOT reach focus on the stars. Nobody cares about auto-focus - its about being able to use lenses that is the issue. Note that the later models have a "floating" sensor that can have the chip position adjusted. This is tricky work to get right and keep orthogonal. At least one of cameras converter guys will not touch these floating sensor cameras. I opted for a replacement filter which adds to the cost but the chip did not need to be moved forward - plus I can use lenses. Astronomiser did the job for me.

Just to confuse things - the 5D MkII is starting to get reasonable(ish) second-hand - now that has big pixels!!!!

Thanks for the info David. I'm looking around for a good buy and of course price comes into it.I put the question about pixel size to the chap at JTW Astronomy (their prices seem ok) Here's his reply.

It's a tricky one. Using rule of thumb you can estimate the 1100D sensor to have a full well capacity of 42500 electrons. The highest number you can count to with 14 bits is only 16383. So there is a bottleneck when the image is digitised anyway. Pretty much all the Canons have a 14 bit ADC. The benefit from large pixels is increased sensitivity, but the more expensive models are less noisy despite their smaller pixels. There are many factors at play but what we have seen is that low noise is the one to shoot for when everything else is similar.

Bill,He is not quite correct. The full well of the 1100D is 32,000. The 40D is 40,000, the 60D is 24,000 and the 5DMkII 64,000. If you set ISO to 400 then you can divide these values down by about 4. so no "bottleneck".As regards camera noise (he means read noise), then we here are very much limited by sky noise. So unless you are shooting 100 x 1 minute exposures (not recommended), read noise will not be the limiting factor. The best way to maximise signal-to-noise is to increase signal - many long sub-exposures.

I don't think any of the Canon cameras from the 350D onwards would be a bad choice - except perhaps the 50D. The smaller pixel models ideal for shorter focal lengths, bigger pixels for longer focal lengths. I am happy with the 60D and its small pixels on a 500mm focal length. The 40D or 1100D would be better for longer focal lengths and the 5DMkII even better still. So it really depends on what you have in mind.

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